The invention described herein pertains generally to organic gels and more specifically to organic aerogels and methods for their preparation.
Foamed organic polymers and organic foam composite materials are well known and are used in the insulation, construction and similar industries. These foams, generally, are of relatively high density and are not suitable for applications where very low density ( .ltoreq. 100 mg/cc) organic foams are needed, such as in many high-energy physics applications, or as parts for inertial confinement fusion targets. Another requirement for such organic materials are not only the very low density, but generally at least over an order of magnitude smaller in cell size than foams produced using other conventional techniques such as the expansion of polymer/blowing agent mixtures, phase-separation of polymer solutions and replication of sacrificial substrates, to name a few. Some of these prior art methods have produced phenol-formaldehyde and phenol-urea foams but again, these foams have a compact cellular structure and densities much greater than the 100 mg/cc level required for high-energy physics applications. The following patents exemplify the phenol-formaldehyde and phenol-urea type of resins produced using the earlier-mentioned conventional techniques.
U.S. Pat. No. 4,694,028, issued Sept. 15, 1987, to Yukio Saeki et al., describes a method for the production of a phenol resin foam exhibiting a compact cellular structure, a high closed cell ratio and improved heat resistance.
U.S. Pat. No. 4,576,972, issued Mar. 18, 1986, to James Lunt et al., discloses closed cell foams produced from modified, low cost phenol formaldehyde resoles. These foams exhibit high closed cell content, low friability and low thermal conductivity.
U.S. Pat. No. 4,546,119, issued Oct. 8, 1985, to James Lunt et al., teaches a foam produced from phenolic resins and a method for the production thereof. These foams have low thermal conductivity and low friability.
U.S. Pat. No. 4,525,492, issued June 25, 1985, to Mary H. Rastall et al., discloses a modified phenolic foam produced from phenol-formaldehyde resins with a phenol to formaldehyde ration of about 1:3 and 1:4.5. These foams are thermally stable, fire resistant and low in cost.
U.S. Pat. No. 4,489,175, issued Dec. 18, 1984, to Heinz Baumann, describes a method for the preparation of an urea formaldehyde combination foam with a low content of formaldehyde which is dimensionally stable.
U.S. Pat. No. 4,417,004, issued Nov. 22, 1983, to Krishnan K. Sudan, teaches a two-part pack for the in-situ production of a phenol formaldehyde foam. The packs contain premixed ingredients which are placed at the site and mixed to produce the foam at the desired site.
While these foams are useful for thermal insulation in the building industry, the methods or the materials produced by these prior art methods do not exhibit the desired low density, the continuous porosity or the ultra-fine cell size ( .ltoreq. 0.1 microns) required for high energy physics applications or as parts for inertial confinement fusion targets.
A few low density foams that have been produced are illustrated in the following patents:
U.S. Pat. No. 4,602,048,issued July 22, 1986, to Harold R. Penton et al., discloses a composition of low density polyphosphazene foam made from gums which are substituted with phenoxy, alkyl phenoxy and alkenyl phenoxy substituents. The preferred alkyl phenoxy groups are those where the alkyl group contains 1-4 carbon atoms. Fillers include inorganic materials such as carbon black or glass fibers.
U.S. Pat. No. 4,595,623, issued June 17, 1986, to Preston S. Du Pont et al., describes a fiber-reinforced, syntactic foam composite with a low specific gravity. The foam is produced by dispersing microscopic particles in a thermosetting composition and then curing. The particles incorporated may be carbon or phenolic resins. Carbon fibers are added to the syntactic foam to improve the strength of the composite.
U.S. Pat. No. 4,465,792, issued Aug. 14, 1984, to Donald G. Carr et al discloses a flexible foam composition in which inorganic reinforcements such as carbon fibers, glass, paper, cloth or woven tape, may be incorporated. Accelerators and promoters, such as commercially available naphthenates may be added to supply metal ions to improve the cure time.
U.S. Pat. No. 4,049,613, issued Sept. 20, 1977, to Dwain M. White, discloses a method for the production of carbon fiber-polyetherimide matrix composites, with high strength and superior solvent resistance. Dihydricphenols can be used to make alkali metal diphenoxides which are then used to make the polyetherimides.
However, these materials do not exhibit the desired low density, the stability or the ultra fine cell structure and are thus not suitable for applications in high energy physics or as parts for inertial confinement fusion targets. The current production of low density materials (less than 100 mg/cc) with ultra-fine pore sizes (less than 1 micron) has largely been limited to aerogel technology, particularly to silica aerogels.
Gels are a unique class of materials which exhibit solid-like behavior resulting from continuous, three-dimensional framework extending throughout a liquid. This framework consists of molecules interconnected through multifunctional junctions. These junctions can be formed through covalent crosslinking, crystallization, ionic interactions, hydrogen bonding or chain entanglements. In some cases, junction formation is reversible and the gels revert to liquid-like behavior upon a change in temperature.
Gels have been synthesized for a variety of applications, and their properties studied for these various applications. The following examples illustrate a few of these applications. Electrophoresis of protein mixtures is quite often performed with crosslinked polyacrylamide gels. Some chromatographic packings are composed of styrene/divinyl benzene gel particles. Hydrogels containing 2-hydroxyethyl methacrylate are used in many soft contact lens applications. Thus, the gel framework is tailored to the specific end use. As an example, the molecular weight between crosslinks partially determines the water content of a soft contact lens.
In other examples, these gels serve only as an intermediate phase in materials processing. High modulus fibers are made by solution spinning a polyethylene/decalin solution, quenching the filaments to form a gel, and stretching to a high draw ratio. Gels produced from the phase separation of dilute polymer solutions serve as precursors to membranes and low density foams.
Sol-gel processing of ceramics has gained considerable attention in recent times because various metal oxides can be formed from gels at relatively low temperatures when compared to conventional melt processing. In the case of silica gels, the solvent is slowly evaporated to form a dry, porous xerogel which can be sintered into a glass. If the solvent is removed from the silica gel by a supercritical drying process, a transparent low density foam results. Foams of this type are referred to as inorganic aerogels. In these systems, solutions of the appropriate constituents are reacted to form colloidal particles which are covalently linked together to form a three-dimensional network. The solvent for the reaction is carefully removed using a procedure such as critical point drying, resulting in a low density aerogel which is translucent because the ultra-fine pore size minimizes light scattering.
Silica gels, generally, are produced from the polycondensation of tetramethoxy silane (TMOS) or tetraethoxy silane (TEOS) in the presence of an acid or base catalyst. Under acidic conditions, linear or slightly branched polymers are formed which entangle and then crosslink to form a gel. Under alkaline conditions, the reaction produces branched polymeric "clusters" which crosslink together. This is similar to gel formation from the destabilization of colloids. The size of the "clusters" and their interpenetration can be manipulated under appropriate reaction conditions.
The small cell size (.ltoreq. 500 .ANG.) of the silica gel necessitates supercritical drying. Large capillary forces at the liquid-vapor interface cause the silica gel to shrink or crack if the solvent is removed by evaporation. In the case of supercritical drying, no surface tension is exerted across the pores, and the dry aerogel retains the original morphology of the silica gel.
Silica gels are unaffected by the solvent which fills their pores. The gel can be exchanged into an organic solvent from its original alcohol/water environment without swelling or deswelling. In gels such as crosslinked polystyrene, on the other hand, the solvent interaction parameter determines the equilibrium swelling behavior.
In spite of the many advantages of these silica aerogel systems, the presence of silicon (Z=14) in the composition, however, often limits its effectiveness for many applications, such as in high energy physics or as parts for initial confinement fusion targets and the like, where a low number for Z (atomic number) is preferred. Pure organic foams or aerogels, consisting of mostly carbon (Z=6), and hydrogen (Z=1) with some oxygen (Z=8), would be suitable for such applications, if stable, low density aerogels of ultra-fine pore size could be produced. However, no relationship or analogy has been stated to exist between silicic acid polymerized in an aqueous system and condensation-type organic polymers.
A need exists, therefore, to produce low density organic foams or aerogels which exhibit continuous porosity gels, for applications in high energy physics, parts for inertial confinement fusion targets, and, with modifications, for other applications such as chemical catalysis and ion exchange reactions.
Accordingly, it is an object of the present invention to provide different types of low density organic aerogels which exhibit continuous porosity and ultra fine cell size of the order of about 0.1 microns.
Still another object is to provide low density organic aerogels with a density of .ltoreq. 100 mg/cc.
Another object of the invention is to provide a synthetic route for the production of the organic aerogels.
Yet another object is to produce organic aerogels from formaldehyde and other phenolic substances.
Still another object of the invention is the incorporation of other organic and inorganic components into the aerogels.
Yet another object is to provide carbonized aerogels of low densities and ultra-fine cell size.
An additional object is to provide modified aerogels which would be suitable for applications in chemical catalysis and ion exchange reactions.
Another object is to provide a low density, inertial confinement fusion targets with Z=8 or less.
Additional objects, advantages and novel features of the invention, together with additional features contributing thereto and advantages accruing therefrom will be apparent from the following description and the accompanying illustration of one or more embodiments of the invention and the description of the preparation techniques therefor, as described hereinafter. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.